The fuel stations around here only sell one type of petrol and one type of diesel for cars. My car takes diesel and one thing I'm fairly sure about is that the diesel mileage does vary from one provider to another and I know a few people who noticed similar variations as me, especially with Texaco.
When the fuel light flashes on my dashboard, it takes 42 litres to fill. I typically get 580 miles (~930km) on a full tank before the fuel warning LED. When I use to purchase fuel at one Texaco station, I got a fairly consistent 550 miles (~885km.) I use to buy my fuel there as it use to also be the cheapest, until one day the fuel prices came down and I ended up filling the tank somewhere else. The Texaco claims they add "Techron" and I've often wondered if they do this to 'dilute' the fuel, as I always got better mileage from diesel bought just about everywhere else.
The longest range I got before the warning is about 610 miles (~980km) and I think that was with diesel from a Topaz station, but then again their fuel is generally the most expensive. The fuel station I use now is at a fuel distributor which only sells diesel and despite being the cheapest, I get the decent ~580 miles on a tank. Another reason I buy there is that an assistant always fills the car, so no annoying diesel smell on my hands either.
As for diesel pricing around here, I pass roughly 10 fuel stations on my hour drive to work. The price currently ranges between 144.9c and 148.9c a litre. From what I recall, the fuel distributor is 142.9c a litre of diesel. I don't know what the petrol prices are since I generally don't check them.